With so many different types of light source available to us today, we tend to forget that for many years people who worked in coal mines had to use candles, using a naked flame with the potential for ignition and explosion if there was a leak of methane.
At that time, one of the most important jobs in the mine was that of “fireman”. His purpose in life was not to put out fires, as we might think today, but to actually ignite small fires so that any methane which collected in the roof of the mine was burnt off before it had a chance to ignite. He passed through the mine workings with a lighted taper on the end of a long pole, deliberately inserting the flame into any pockets in the roof where the methane (at about half the density of air) would collect. Rumour has it that he was either the best paid man in the mine or, using a modern phrase, a “prisoner on day release”.
It may seem a crude process, but we still follow this principle today. The commonly-seen flare stacks from oil platforms and refineries are doing exactly the same job, burning the methane (and sometimes other gasses) in a safe manner so that they can’t form a gas cloud which could be ignited and cause a major explosion.
In 1815, Sir Humphrey Davy devised the first “flame safety lamp”, although it would take several years for manufacturers to make safety lamps in the form we would recognise today. This transformed “task lighting” in the mining industry, and was the first source of light designed specifically for use in hazardous areas. Even when battery-powered caplights became the norm, the flame safety lamp continued to be used for many years, because of it’s reliability and for it’s ability to act as a gas detector (the flame changing shape and colour if gas was present).
Thus historically, the protection of electrical equipment against becoming a source of ignition of a gas/air mixture was developed in the mining industry. The worst disaster in the UK, where over 400 lives were lost, was in 1913 at the Senghenydd Colliery in South Wales. It is thought the most probable cause of ignition was a spark from the bell signalling system, and from the subsequent investigation grew the technique that we know today as Intrinsic Safety.
“the protection of electrical equipment against becoming a source of ignition of a gas/air mixture was developed in the mining industry”
By carefully designing the circuit, it is possible to ensure that any residual level of energy in an electrical spark is insufficient to ignite the atmosphere, and that all the individual component temperatures are below the auto-ignition temperature of the atmosphere. Standards were developed separately in many countries, but eventually today we have the internationally-recognised standard, IEC 60079-11.
Intrinsic Safety is only appropriate for comparatively low power circuits (generally less than 3 watts, but varying with supply voltage), and even the ubiquitous miners’ caplight with lead-acid battery, which has been used for so many years, does not quite meet the standard. Some additional precautions have to be taken according to the specific standard for such caplights. However, the development of high power LED light sources has provided the technical development necessary for caplights to be manufactured in accordance with the IEC 60079-11 standard and fully meet the requirements for Intrinsic Safety.
“the development of high power LED light sources has provided the technical development necessary for caplights to be manufactured in accordance with the IEC 60079-11 standard and fully meet the requirements for Intrinsic Safety.”
The flameproof protection concept was developed at the same time, and is based on having a sufficiently strong enclosure which can both withstand an internal explosion and vent the products of combustion to the external atmosphere in a way which will not ignite it. Although non-metallic flameproof enclosures are possible, the equipment tends to be characterised by heavy cast or fabricated metal construction. An added problem for portable equipment is that it is not easy to provide protection when batteries are incorporated, and the possibility of an oxygen-enriched internal mixture has to be considered. The current standard is IEC 60079-1.
“An added problem for portable equipment is that it is not easy to provide protection when batteries are incorporated, and the possibility of an oxygen-enriched internal mixture has to be considered.”
The concept of Increased Safety was developed later, originally in Germany but eventually finding favour in most of the world (excluding North America) in the 1960s. The principles outlined in IEC 60079-7 require that equipment which is non-sparking in normal operation is constructed with sufficient safety margins to ensure that failures are most unlikely and that high temperatures do not occur. Electrical segregation is increased and better quality insulation is specified. Terminations must have special features to prevent inadvertent loosening and the resultant sparking or heating.
For many years, the most favoured form of protection for handlamps and torches has been a combination of Intrinsic Safety and Increased Safety. Such lamps are characterised by a strong lens, often with additional mechanical protection in the form of a raised rim, and the need to use some form of tool to access the battery compartment to change the battery.
If you work for one of the emergency services, you will probably be provided with this type of lamp or torch, so that you don’t have to worry about going into areas where gas might be present. Examples can be seen on television programmes featuring police or firefighting activity.
It is very important that the user adheres to instructions regarding the types of cells which can be fitted. If the protection relies on the inherent reduced short-circuit current of a zinc-chloride cell, introducing an alkaline cell can violate the safety. When delivering training courses, I often ask people to estimate the short-circuit current of a “D” size alkaline cell as might be used in a torch. Most are surprised to learn that the current is usually somewhere between 30 and 40 amps. That is a lot of current if delivered directly into a deliberate or inadvertent switching contact.
Increasingly we are seeing rechargeable cells, which bring additional problems of electrolytic gas production, and the very special safety issues of some of the lithium-based batteries. For Intrinsic Safety, the standard requires that the instantaneous short circuit current is measured with any safety devices removed from the circuit. Most lithium cells have protection built into the cell, for example a “pill” of PTC resistive material so that a hot cell can only produce a reduced current. For test purposes, we have to convince the cell manufacturer to let us have special cells with this protection removed. The safety case for the test method is extensive!
However, LED technology has certainly made life easier when constructing hand-held lighting. There is now no need to follow the “Increased Safety” approach to protecting the hot wire in the bulb, so the lamps can be made smaller and lighter. Battery life is extended, or a smaller battery can give the same life.
“LED technology has certainly made life easier when constructing hand-held lighting”
When considering miners’ caplights and other similar applications, the light output for a defined period is important. A caplight which went out before the end of the shift due to battery exhaustion would be useless. It has become practical to mount the smaller batteries within the headpiece for some applications, doing away with the need for the separate belt-mounted battery and intervening cable.
“It has become practical to mount the smaller batteries within the headpiece for some applications, doing away with the need for the separate belt-mounted battery and intervening cable”
But LEDs do introduce a new problem. It has been known for many years that the output from high power laser diodes, such as used for driving signals through optical fibre cables, are capable of causing ignitions. Some gasses will absorb the radiated power directly and ignite, whereas all gas/air mixtures are susceptible to ignition from dust particles which have entered the beam and glow white hot.
Many of the high power LEDs which have come on the market recently are capable of providing an equally intense beam, so additional precautions must be taken. For most purposes, the distance between the actual light source, such as a hot wire filament or a discharge tube and the outside of the bulb glass, has been sufficient to keep the beam intensity below critical levels, but the LED does not usually have that additional safety factor built in. The ability to focus such a small intense source can also be problematical. More information is contained in IEC 60079-28.
Hand-held or portable lighting is always regarded as requiring protection suitable for use in Zone 1, since it is likely that it might be used in any hazardous area. Fixed lighting, however, can be designed for the particular Zone of use.
“It is the legal responsibility of the owner or operator of a plant or facility to carry out the Zone Classification exercise, based on an engineering judgement of the possible sources of release of the flammable material.”
Hazardous areas are divided into Zones (see below), based on the likelihood that an explosive atmosphere may occur. It is the legal responsibility of the owner or operator of a plant or facility to carry out the Zone Classification exercise, based on an engineering judgement of the possible sources of release of the flammable material. Guidance is provided in IEC 60079-10 and in many industry specific guides, such as IP15, from the Energy Industries Council, which is applicable to the petroleum industries.
Zone 2 is the least hazardous area, and the explosive atmosphere will not occur during normal operation and, if it does, it will exist for a short time only.
Zone 1 is an area where an explosive atmosphere may occur occasionally in normal operation, though not for protracted periods.
Zone 0 is an area where the explosive atmosphere is likely to occur in normal operation and may be present continuously.
It is highly unlikely that sources of illumination will be installed directly in Zone 0 and then they must be protected to the highest level of Intrinsic Safety. If significant illumination is required in Zone 0, it would be normal for the actual lights to be mounted in the adjacent Zone 1, with the beam shining into the Zone 0 area, possibly through a glass dividing wall.
It is sometimes regarded as sufficiently safe to use portable Zone 1 lighting taken into Zone 0 for a very short period. An example might be the use of a Zone 1 type caplight or torch by an operator wearing breathing apparatus while inspecting the inside of a large tank or process vessel.
“For high power lighting requirements (usually high power discharge lamps) there are only two practical forms of protection.”
For high power lighting requirements (usually high power discharge lamps) there are only two practical forms of protection. In Zone 1, the luminaire is almost always flameproof, with the assumption that if gas or vapour gets into the fitting, the resultant explosion will be safely contained.
In Zone 2, while the use of the heavy and expensive flameproof protection is often used, the more elegant solution is to use the cheaper and lighter form of protection known as Restricted Breathing. In this case, the high temperatures are contained in an “almost” sealed enclosure which will not allow an interchange of atmosphere during the short time that a flammable gas or vapour is present outside the enclosure. The fundamental test for this is to pull a vacuum on the enclosure and ensure that the half-life is more than three minutes. Depending on the nature of the enclosure, there may be a test point for use during installation and routine inspection, to prove that the Restricted Breathing properties are effective.
Restricted Breathing is one of the concepts contained in IEC 60079-15, the standard for Ex “n” protection which is a compendium of requirements for different types of protection which are only suitable for installation in Zone 2.
The fundamental philosophy of Ex “n” is that the equipment is non-sparking in normal operation, with just a few minor additions to the requirements that would apply to normal industrial equipment of a similar design.
“The fundamental philosophy of Ex “n” is that the equipment is non-sparking in normal operation”
For luminaires, the additions are concerned with the temperatures that might occur on failure of a lamp. This is particularly important for fluorescent lamps where, under some circumstances, it is possible for the control gear to run much hotter with a partially failed lamp than with a healthy lamp. This is because the cathode emitter material will erode at different rates from either end of the lamp, resulting in partial rectification and a d.c. current which is not well controlled by the gear.
Increased Safety luminaires, suitable for installation in Zone 1, are almost invariably fluorescent, although GLS tungsten lamps are permitted provided that the lamp glass temperature is cool enough. Originally the fluorescent versions used the unique TLX tube arrangement.
The tubes were designed to start without pre-heating of the cathodes and usually required a semi-resonant combination of inductors and capacitors to provide sufficient voltage, despite having a semi-conducting strip along the outside of the tube to assist formation of the initial discharge. They were also provided with a unique single-pin lamp cap which matched the pseudo-flameproof socket. The absence of a pre-heating circuit ensures that no heat is produced by a broken lamp and that the high temperatures at the cathodes dissipate in a very short time if the tube breaks while under power. Conversely, a normal pre-heat circuit is acceptable for Ex “n” luminaires.
The advent of electronic ballasts has had a radical effect on the design of the luminaires, and it is now quite common to use standard bi-pin tubes in an Increased Safety luminaire where the special ballast will provide a sufficiently high voltage pulse to initiate the discharge. The high voltage pulse does, of course, pose its own problems in terms of voltage withstand tests, etc. In these arrangements, both pins are connected directly in parallel at the lampholder, thus ensuring that there are two parallel paths for the lamp current, either of which would be adequate if there were a bad connection in the other.
“The latest developments are targeted at lower maintenance and higher efficiency.”
The latest developments are targeted at lower maintenance and higher efficiency. For flameproof luminaires, there are few restrictions on the light source, but it is at the expense of a heavy engineered enclosure. Attempts have been made to use the QL lamp in Increased Safety luminaires, but the hollow centre of the actual lamp can be open to gas ingress and the temperatures at that point are higher than most people would expect. Clearly, at the moment, the future belongs to high power LEDs.
“Clearly, at the moment, the future belongs to high power LEDs.”
Almost inevitably, the LEDs themselves need additional protection by encapsulation to IEC 60079-18, with the encapsulated block normally mounted in an Increased Safety fitting. Heat dissipation from the LED is an issue that needs careful design. The total power to be dissipated may be less than with older types of lamp, but the concentrated point sources can lead to high temperatures if inadequate precautions are taken.
Getting the design of the lighting correct is only one of many issues to be considered. In Europe, our legal framework is controlled by two directives: 94/9/EC, the ATEX Product Directive; 1999/92/EC, the ATEX User Directive.
The two directives work together to try to ensure that only appropriately designed and manufactured equipment is installed in hazardous areas, and that the installation methods, inspection, maintenance and repair of the equipment is adequate. The “User” directive leans heavily on the use of “competent” people to do the work.
“The two directives work together to try to ensure that only appropriately designed and manufactured equipment is installed in hazardous areas, and that the installation methods, inspection, maintenance and repair of the equipment is adequate.”
The Product Directive
For the “Product” Directive, the different Categories of Equipment require different conformity assessment regimes:-
Category 1 Equipment requires the highest level of control and is suitable for installation in all Zones.
Category 2 Electrical Equipment must also have certification from an ATEX Notified Body, but the same stringency does not apply to Category 2 Non-Electrical Equipment.
Category 3 Equipment does not need independent certification and the manufacturer is allowed to declare compliance without involving any outside body.
In every case, the equipment must be accompanied by the manufacturer’s Declaration of Conformity as outlined in Annex X of 94/9/EC.
There is a body of opinion that the ATEX Directives do not always provide sufficient confidence in the equipment, particularly as the only mandated document to be passed to the purchaser is the manufacturer’s own declaration, and that for much equipment there is no involvement of an expert third party.
The IECEx Scheme
To answer these issues and to provide worldwide acceptance of equipment, it is necessary to turn to the IECEx Scheme. This is a proper certification scheme, with the certification body responsible for issuing the top document, the IECEx Product Certificate, and all of the types of equipment being subjected to the same rigorous conformity assessment regime.
“To answer these issues and to provide worldwide acceptance of equipment, it is necessary to turn to the IECEx Scheme.”
Furthermore, the IECEx Certificate can be viewed on-line by anyone with internet access, thus giving total transparency. All of the certification bodies in the scheme have been admitted following a peer assessment, involving experts from three separate countries. It is part of the philosophy that a certification body cannot be assessed by anyone from their own country. All the IECEx Certification Bodies located in Europe are also Notified Bodies for the ATEX Directive, but there are many Notified Bodies that are not in the IECEx Scheme.
As part of the development of the entire IECEx System, certification schemes have also been devised to cover “Service Facilities”, that is the workshops which service and repair equipment, and Personnel Competency. This latter scheme is only just starting but it has been established that there is a large market for a scheme which provides international confidence in the competency of named individuals working with hazardous area equipment.
History has shown that all recent major explosions have been down to actions of people. We have solved the technical issues which caused events like the Senghenydd Colliery disaster. It is now clear that by far the weakest link in the safety chain is the competence of the individual people involved in the installation, inspection, maintenance and repair of the hazardous area equipment.
“It is now clear that by far the weakest link in the safety chain is the competence of the individual people involved in the installation, inspection, maintenance and repair of the hazardous area equipment.”
IECEx is determined to provide a complete global solution for plant operators wanting to operate safely. For more information on this and the other IECEx Schemes, visit www.iecex.com
Ron Sinclair is Managing Director of Baseefa, the foremost certification body in terms of international certification of Hazardous Area equipment, having issued more internationally accepted IECEx product certificates than any other body in the world.
He is chair of ExTAG, the committee of the international certification bodies within the IECEx Scheme and also a vice-chair of the equivalent meeting for European Notified Bodies under the ATEX Directive. Ron is also chair of both the British committee GEL/31 and the European Cenelec Committee TC31, responsible for the writing of standards for electrical equipment in Hazardous Areas.
In addition to product certification for IECEx and ATEX, Baseefa offers Service Facility Certification and Personnel Competency Certification within the IECEx System. For plant owners and operators, Baseefa offers its DSEAR compliance service to assist compliance with the requirements for the use of equipment. All this is backed up by a comprehensive training programme, making the knowledge accumulated by its staff over a total of more than 300 years’ experience available both through general courses and specific courses developed for individual customers.
For more information contact either [email protected] or [email protected] or visit our website at www.baseefa.com
Baseefa Ltd Staden Lane Buxton Derbyshire SK17 9RZ Phone 01298 766600 Fax 01298 766601
Published: 10th Apr 2010 in Health and Safety International